Effect of non-metal doping on thermodynamics of dehydrogenation in MgH2

Abstract

MgH${}_2$ is a potential hydrogen storage material for future applications due to its high abundance, gravimetric and volumetric hydrogen storage density compared to other alternatives of metal hydrides. However, high desorption temperature is a major challenge that hinders its application. The present study investigates the effect of site-selective non-metal doping (2p-series: B, C, N, O, and F, and 3p-series: Si, P, S, and Cl) on formation enthalpy and structural stability of doped-MgH${}_2$ using density functional theory (DFT) based calculations. Screening factors such as desorption temperature, bulk modulus, and gravimetric density are considered for screening the dopant atoms, lowering the desorption temperature. Our screening finds doping B, C, N, Si, P, and S reduces the desorption temperature while preserving similar gravimetric density and Bulk modulus as of pristine MgH${}_2$. Contrarily, doping of F, Cl, and O increases the desorption temperature. The C is the most composition-sensitive dopant among all. However, in B, C, Si, P doping, the change in bulk modulus is minimum ($<$ 5%) compared to the pristine MgH${}_2$. Moreover, these dopants have the highest impact on the desorption temperature, making them the most preferable dopants among the list. A strong negative correlation between the electronegativity of dopants and the formation enthalpy is found for interstitial/H-substitutional doping sites. A detailed electronic structure analysis points out that the non-metal doping at interstitial/H-substitutional sites destabilizes the Mg–H bonds are suitable candidates to reduce the desorption temperature.